Because it is against the law to tamper with fire equipment, including
smoke detectors, and because smoke detectors are usually centrally
located and unobstructed, employeres wishing to hide a camera in an
employee locker room have often put it in the smoke detector.
In fact there are companies who sell smoke detectors with built in
cameras.

These cameras should be against the law for the same reason that
tampering with smoke detectors is against the law. Such cameras
encourage tampering with smoke detectors, obstructing smoke detectors,
and the like, and result in increased fire hazards even where they
are not used, simply because of suspected use.